As is known in the art, GaN HEMT devices require insulating buffer layers for optimal performance. Unmodulated current flowing deep in the buffer layer will degrade output power and efficiency. FIG. 1 illustrates a typical GaN HEMT structure grown on an insulating SiC substrate. An aluminum nitride (AlN) nucleation layer is first grown on the substrate since gallium nitride, grown directly on SiC will exhibit a significant conductivity spike at the GaN/SiC interface. AlN has a very large bandgap (6.3 eV) which facilitates high resistivity.
However, even with an AlN layer we have observed conductivity spikes in HEMT material. FIG. 2 shows a doping-thickness plot of a sample which had an AlN nucleation layer with thickness of 1350 Å. The plot exhibits a conductivity spike deep in the material at a depth of 104 Å or 1 μm. The reverse bias leakage current was also measured on this sample using a mercury probe system. As shown in FIG. 3, the leakage current was significant (20 amperes/cm2) at −80 volts. The AlN in the sample shown in FIG. 1 was grown with an Al/N ratio of approximately 1.57, where the ratio Al/N is the ratio of Al to reactive nitrogen, it being noted that much more nitrogen flux is used compared with the aluminum flux, but only a small portion of the nitrogen flux is reactive. This ratio of Al to reactive nitrogen indicates that there was approximately 57% excess on the growth surface of aluminum to reactive nitrogen. It has been empirically found that Al to reactive nitrogen ratios greater than 1 (aluminum-rich) result in improved films.
FIG. 4 shows the SIMS (secondary ion mass spectroscopy) profile of the sample of FIG. 1. The N-type conduction spike is caused by silicon from the SiC substrate which is migrating through the AlN layer and piling up at the AlN/GaN interface. GaN is easily doped by silicon which results in the conductivity. The sample shown in FIG. 1 contains a 1350 Å AlN layer grown with Al/N=1.57. The profile shows silicon from the SiC substrate migrating through the thick AlN layer and piling up at the GaN/AlN interface.
We believe that the mechanism for such rapid silicon diffusion through the thick AlN layer is due to the excess aluminum on the surface which reacts with the SiC substrate. At our growth temperature of approximately 750° C., aluminum is a liquid.Al(liquid)+SiCAl4C3+Al(liquid+Si)  (1)Since silicon is now in a liquid state, it can rapidly move through the AlN film which is grown with excess aluminum.
Now that we have discovered the source of the doping (silicon) and mechanism for the rapid dopant migration, a structure is provided having a two-step AlN nucleation layer. In the old process the silicon concentration in the AlN peaked at greater than 1×1020 cm−3. In the process according to the invention, the silicon concentration peaked at less than 3×1018 cm−3. Thus, in accordance with the invention the AlN is substantially free of silicon.
In accordance with the invention, it has been demonstrated that good AlN material quality may be obtained with the entire AlN layer being conventionally grown by molecular beam epitaxy (MBE) with an Al to reactive nitrogen ratio greater than 1 (aluminum-rich).
In one embodiment, an initial AlN layer is grown with an Al to reactive nitrogen ratio less than 1 (nitrogen-rich) so that there is no free aluminum to react with the SiC surface. Once the SiC surface has been completely covered by AlN, the Al to reactive nitrogen ratio is increased to greater than 1 (aluminum-rich) for the rest of the layer to improve the material quality. The initial layer can be thin (30–200 Å) since its function is to cover the SiC surface. By making this layer thin, the roughness associated with growing AlN nitrogen-rich is minimized. The second AlN layer grown Al-rich improves the material quality.
In accordance with the invention, a semiconductor structure is provided having a substrate, a first AlN layer having an Al to reactive nitrogen ratio less than 1 disposed on the substrate, and a second AlN layer having an Al to reactive nitrogen ratio greater than 1 disposed on the first AlN layer.
In one embodiment, the substrate is a compound of silicon and wherein the first AlN layer is substantially free of silicon.
In accordance with the invention, a method is provided for forming a semiconductor structure comprises: growing a layer of AlN on a substrate comprising a compound of silicon with the reactive nitrogen flux greater than the aluminum flux; and changing the aluminum and reactive nitrogen fluxes such that the aluminum flux is greater than the reactive nitrogen flux.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.